Visible laser projection systems are now widely used in the industry to project a laser outline, laser image or laser template on a target surface or workpiece for assembling large two or three-dimensional structures or assemblies, such as prefabricated roof trusses and aerospace composite components. By precisely characterizing the laser projector and establishing the exact relative position of the laser projector to the assembled structure, composite or target workpiece, the laser projection system is capable of accurately producing a laser image or template at precisely known coordinates on a target surface or workpiece which may be planar or curvilinear. For example, U.S. Pat. No. 5,646,859 assigned in part to the assignee of this application discloses a method and apparatus for defining a laser template for assembling a structure, such as a prefabricated roof truss. The method and apparatus disclosed in this patent includes a laser projector or a plurality of laser projectors mounted above a work surface, a plurality of sensors or laser targets fixed at predetermined locations on or adjacent the work surface, a computer and a sensor on the laser projector. The laser projector periodically or continuously scans the laser targets and the reflected light from the laser targets to the sensor of the laser projector determines the precise projection angle associated with the center of each laser target datum. Using a series of mathematical algorithms, the precise position and orientation of the laser projector relative to the work surface or workpiece is then calculated by the computer. The spatial information, in conjunction with a known display list or data stored in the computer allows the laser projector to generate accurate laser templates or laser images on the target surface. The laser projector may be fixed relative to the work surface or for larger assemblies, a plurality of laser projectors may be used or the laser projectors may be moved relative to the work surface as disclosed in the above-referenced patent.
Alignment and calibration methods similar to the above provide the accuracy needed for a wide range of laser projection applications. A typical accuracy specification is ±0.015 inches at a 10 to 15 foot stand-off of the laser projector when measured perpendicular to the laser beam. This approach allows good flexibility in positioning the laser projectors because the mounting location can be arbitrarily selected so long as a sufficient number of known laser target locations are detectible within the field of view of the laser projector which, as set forth above, must be located at a predetermined location on or adjacent the target surface. In a typical application, a minimum of four laser targets must be located by the sensor system (laser target and sensor) to establish the position of the laser projector relative to the assembled structure or part and the target surface.
3D digitizer scanners or 3D digitizing systems are also commercially available which projects patterns of light on a target surface, part or assembly in rapid sequence and a digital camera records these patterns and the images are then processed by combined Gray Code/Phase shift technique to compute a dense cloud of three dimensional coordinates for the surface of the part being measured. A conventional 3D digitizing system includes one or more digital cameras and generally includes a source of light, typically a white light, which may be rigidly mounted or mounted on a robot arm or the like. The digital scanner produces an accurate dense point cloud of the target surface, part or assembly which can be used for reverse engineering applications, inspection or the like. However, the digital scanner also requires photogrammetry targets at predetermined known locations on the target surface, part or assembly to determine the precise position and orientation of the digital scanner relative to the target surface, part or assembly.
Pending U.S. patent application Ser. No. 10/913,842 filed Aug. 6, 2004 assigned to the assignee of this application discloses a method and apparatus for independently determining a position and orientation of a target surface or workpiece using an external metrology device, such as an indoor global positioning system as disclosed, for example, in U.S. Pat. Nos. 6,501,543 and 6,535,282, eliminating the requirement for laser targets on the workpiece within the field of view of the laser projectors. The laser projectors are mounted on or in a frame assembly having a plurality of metrology receivers and metrology receivers are positioned on or adjacent the workpiece at predetermined known positions, such that a metrology transmitter can determine the precise location and orientation of the laser projectors relative to the workpiece in three dimensions. This method and apparatus may then be used to very accurately project a laser image or laser template on a large workpiece, such as an aircraft, and permits relative movement of the workpiece and the laser projectors.
As will be understood by those skilled in this art, a conventional laser projection system includes one or more laser projectors, a computer, and a laser control file or laser image file to be projected on the workpiece at predetermined precise locations on the workpiece. This laser control file is generated using data in the “as-designed” condition of the workpiece, such as an aircraft, and is used to “steer” the laser projector(s) above the workpiece to generate a three dimensional template for painting insignias, placement of decals, assist mechanical assembly, or build up composite parts. However, in certain applications, the “as-built” condition of the workpiece differs from the “as-designed” condition to such a degree that the laser image projected by the laser projector(s) will not be positioned correctly on the workpiece. For example, an aircraft may be modified during assembly by extending the length or other modifications may be made which require correction of the data of the as-designed condition for accurate positioning of the laser image on the workpiece in the as-built condition. This can be a serious problem particularly, but not exclusively, for larger workpieces subject to modification to accommodate customer requirements.
The laser projection system including an intelligent data correction system and method of this invention solves this problem by comparing the data of the as-designed condition to the as-built condition of the workpiece and modifying the data in the computer of the as-designed condition as required for accurate placement of the laser image or images on the workpiece in the as-built condition without requiring laborious input of further data of each modification to the workpiece made during assembly.